pulse oximetry Flashcards
what is a pulse oximeter?
commonly used non invasive device that measures the % of oxyHb compared to total Hb in pulsatile blood. It uses Spectrophotometry. to do this.
how does a pulse oximeter work?
Relies on the fact that oxyHb and deoxyHb absorb light maximally at differnet wavelengths.
A small probe with 2 LED is placed on a patients finger. These LED transmit light at 660nm (red) and 940nm (infrared).
There is a photodetector the other side of the probe that measures the amount of light. by comparing the light emitted to that detected, the amount absorbed can be deduced using beer and lamberts law.
The LED are flashed in sequence one on, while other is off and vice versa and a phase where both are off.
the photodetector will compare absorption at each wavelength and send the information to a microprocessor.
microprocessor has stored data that can compare the absorbance to that of healthy individuals via an algorithm to determine a level of saturation.
what is the isobestic point?
the wavelength at which both oxyHb and deoxyHb absorb equal amounts of light
e.g. 805nm and 590nm
what is beers and lamberts law? how do these relate to pulse oximetry?
beers law state the light absorbed is proportional to the amount of substance present in the path the light is passing through e.g. the concentration of the Hb. however it is a comparison of oxy and deoxy absorptions, rather than total Hb
lamberts law states the light absorbed is proportional to length of the path the light travels through - hence more is absorbed in systole when more volume of blood in the vessel.
what is the equation for beer and lamberts law?
Absorbance = conc x length x molar extinction coeficient
A=elc
molar extinction coeficient = specific for each molecule at a particular wavelength
how does the computer analyse the absorption ratios to give a % saturation?
absorption at 660nm and 940nm is compared to give a ratio
can compare this ratio to pre-determined data taken from healthy individuals at varying saturations between 70% and 100% (graph ratio vs sats)
therefore not validated below 70%
why are 2 wavelengths used?
940nm = oxyHb absorbs more here
660nm = deoxy Hb absorbs more here.
2 wavelengths are used to compare amount absorbed by each to determine a ratio of oxy and deoxy
what is the role of the computerised unit/ microprocessor?
responsible for causing LED to switch on and off repeatedly
receives output from photodetector during different phases of light emission e.g. at 940, 660, and when both are off.
compares absorption ratios at these and removes the absorptions when both off (ambient light).
it compares ratios to stored data from standards to determine saturation
sends to display.
what are the methods for measuring someones oxygen saturations?
oxygen saturations can be measured invasively and non-invasively
the most common and practical method is the non-invasive pulse oximeter
invasive methods include arterial blood gas analysis using CO-oximetry
pros and cons of pulse oximetry?
pros:
simple, cheap, portable and convienient
gives info on sats + HR + some info on arterial waveform
non invasive
continous reading
cons:
- not validated for <70%
- does not tell you about total Hb hence O2 carrying capacity could still be low if anaemic even with 100% sats
- inaccurate with low perfusion states or arrhyhmias
- slight delay around 10 -20 seconds
- can give false results with certain forms of Hb
what are the sources of error when using pulse oximetry?
patient
* movement, shivering
* hypoperfusion
* arrthymias
* nail polish
* pulsatile venous blood e.g. tricuspid regurg
Hb forms
* carboxyHb - similar absorbance pattern to oxyHb and hence can give falsely high sats
* methamoglobin gives sats of around 85%
equiptment
* interference ambient light and diathermy
medications
* those that cause methamoglobinaemia e.g. prilocaine
* methylene blue gives sats of 65%
what are the components of a pulse oximeter?
probe that fits onto patient finger or ear or nose
one side of probe has 2xLED, the other has a photodetector
microprocessor
cabling
display unit - waveform, HR and sats
how does pulse oximeter reduce impact of ambient light ?
the LED lights are pulsed on and off alternatively but also a phase where both is off such that the absorbance during this phase can be accounted for as this will be the absorbance of any ambient light.
can substract this data from both other data sets to remove the impact of ambient light.
is the pulse oximeter affected by anaemia?
no
it compares absorbance at 940 to 660 and removes any common absorbance and hence comparing only deoxy/oxyHb not total Hb
Draw a graph to demonstrate the absorbance spectra for Oxy and deoxy Hb…
point out 660, 940 and 804 isobestic points
the y axis = log absorbance
how is only systolic blood measured by pulse oximetry?
the pulse oximeter works with pulsatile blood, this is because it substracts that absorbed in diastole away from that absorbed in systole
this is due to lambers law as during systole the blood volume is larger hence path is larger so absorbance is larger.
this allows it to remove constants in absorption by other tissues
the rest of the absorbance is constant , systolic portion is variable
how would you draw the absorbance for methamohaemaglobin on the absorbance curve
same absorbance to deoxy at 660nm
then drops but then rises again such that the absorbance at 940 is the same as it is at 660nm
at 940nm its absorbance is above both oxy and deoxy
